There is great interest in tagging various substances such as explosives, petroleum products and drugs to enable retrospective identification. The most practical taggants are code-bearing microparticles which may be so small as to be virtually invisible to the naked eye. U.S. Pat. No. 3,772,200 describes such a microparticle containing tagging elements in various combinations and concentrations to provide codes which can be interpreted using an electron microprobe analyzer. U.S. Pat. No. 4,053,433 and German Offenlegungsschrift No. 26 51 528, laid open May 12, 1977, describe organic microparticles encoded by sequential arrangements of visually distinguishable colors. U.S. Pat. No. 3,772,099 describes microparticles in the form of conglomerations of selected combinations of line-emitting phosphors which can provide a large number of codes. Meloy U.S. Pat. No. 3,861,886 suggests encoding particles for retrospective identification by atomic absorption spectroscopy, emission spectroscopy, X-ray fluorescence analysis, neutron irradiation, activation analysis, etc.
If any one of these schemes is adopted worldwide, it may become necessary either to establish an international clearing house for the codes or to provide some means for determining which country is involved and where in that country information can be obtained converning the codes. In many cases there will be an urgent need for the immediate decoding, as where a law-enforcement agency wants information relating to an illicit use of an explosive.